1. Field of the Invention
The present invention relates to methods of measuring the optical characteristics of volume holographic gratings with a large spectral coverage.
2. Description of the Related Art
Characteristics of volume holographic gratings are well known. When the Bragg condition, λr=2n(λr)Λ cos(θr), is satisfied the grating will diffract the incident beam into an output beam where λr is the readout wavelength in vacuum, n(λr) is the refractive index of the volume holographic grating element material at the readout wavelength, Λ is the grating spacing, and θ is the readout angle of incidence inside the material. The paper by H. Kogelnik (“Coupled Wave Theory for Thick Hologram Gratings”, H. Kogelnik, The Bell System Tech. J. 48:9, 1969) provides details of volume holographic grating diffraction. For a simple uniform grating, the characteristics are completely determined by the thickness of the volume element, the refractive index modulation depth, the grating spacing, and the slant angle relative to the surface normal. For mass production of volume holographic grating elements it is desirable to measure these characteristics at the wafer level before dicing into final parts of smaller size.
U.S. Pat. No. 7,359,046 discloses methods to measure the optical characteristics of volume holographic gratings (VHG). The method relies on the concept of a collimated laser source 200 and a volume holographic grating 230 placed on a rotation stage 220 (FIG. 1). By using a fixed laser wavelength whose value is below the normal incidence Bragg wavelength of the VHG, the optical characteristics of the VHG can be determined by rotating the VHG. High spectral and spatial resolution has been shown with this method.
The inconvenience of the method disclosed in U.S. Pat. No. 7,359,046 is that it requires single frequency laser sources. It is difficult and very expensive to find laser sources covering a wide spectral range such as for example UV to infrared (300 nm to 4000 nm).